1. Field of the Invention
The present invention generally relates to an improved heated tool for use in welding machines used to join thermoplastic sheets together. More particularly, the invention relates to an improved heated wedge which is designed to advantageously vary the amount of heat transferred to the sheet across the width of the weld.
2. Description of the Prior Art
The welding together of overlapped thermoplastic sheets by heating the upper surface of the lower sheet and the lower surface of the upper (overlapping) sheet, above their melting point, and then applying pressure so that the two melted surfaces join and fuse into a weld is a well established and documented practice. One of the earliest patents in the U.S. is U.S. Pat. No. 4,146,419, Inventor Wolfgang Neidhart, in which many features of currently used welding machines are described. This type of welding machine is widely used in the pollution control industry which lines landfills and other waste containment facilities with chemically resistant and durable thermoplastic sheet liners known as geomembranes.
The most widely used method of heating the surfaces of the sheets to be welded is by dragging a heated block of metal between the overlapped sheets. This block is usually heated by imbedded electrical resistance heaters, but can be heated by induction heating. This block is usually made in the form of a wedge, the wedge tapering to its thin end just ahead of the device which squeezes the two sheets together, usually by means of a set of opposing pinch rollers. It is common to produce two parallel weld tracks to facilitate air pressure testing of the weld seam.
The heating can also be achieved by a jet of hot air (or gas) being blown through a nozzle inserted between the overlapped sheets. This method is less favored because the low specific heat of air and the impracticality of blowing high volumes of air, means the only way of providing sufficient energy to melt the sheet surfaces is to raise the temperature of the air to a level which is damaging to the thermoplastic molecules, causing unwanted degradation. Detailed descriptions of so called Hot Wedge welding can be found in:xe2x80x9cHot Wedge Fusion Welding of HDPE Geomembranesxe2x80x9d by Gary M Kolbasuk Published in xe2x80x9cThe Seaming Of Geosyntheticsxe2x80x9d edited by R. M. Koemer, published by Elsevier Applied Science ISBN 1 85166 4831 and;xe2x80x9cConsistent Wedge Welderxe2x80x9d by Fred Struve in xe2x80x9cGeosynthetic Liner Systems: Innovations, Concerns, and Designxe2x80x9d Published by Industrial Fabrics Association International ISBN 0 935803 01 7.
Until the present the welding parameters of concern have been the temperature of the heated wedge, the speed of welding, the force between the pinch rolls, and in addition the question of using a radiused shape for the heating surface, as opposed to a flat shape has been considered.
Long term durability tests and analysis of the welds have demonstrated that there is a temperature gradient across the heated-track, which becomes the weld-track. The edges are, relatively, the coolest, and the temperature increases to a maximum in the center of the track. Physically this translates into there being more molten material towards the center of the track. As the heated-track passes between the pinch rollers which force the two molten surfaces together some of the molten material is squeezed out of the forged weld zone and shows up as what is known asxe2x80x9csqueeze outxe2x80x9d at each edge of the weld-track. The fact that the center of the track has more molten material, which is squeezed out, is demonstrated by measuring the thickness of the weld across its width. Invariably the center is thinnest and the thickness gradually increases towards each edge of the weld-track. Analysis of this phenomenon by the German Federal Institute for materials development and testing has reported this and made recommendations regarding wedge temperatures, welder speed and pinch roll force, to keep the amount of thickness reduction within a predetermined safe range. The technical paper which reports this isxe2x80x9cAssessment of Seam Quality and Optimization of the Welding Process of HDPE Geomembranesxe2x80x9d by G. Lueders of the Bundesanstalt fuer Materialforschung und pruefung, Berlin, Germany, Published in the Proceedings of Sixth International Conference on Geosynthetics in Atlanta, March 1998
In a primary embodiment, the invention provides for an improved heated wedge, in which each heating surface has at least one area of reduced heat transfer efficiency for some distance along its length. These areas of reduced heat transfer reduce the amount of heat transferred from the wedge to those parts of the sheet which pass directly over them. By positioning such areas of reduced heat transfer efficiency centrally across the width of the wedge heating-track, and designing such areas"" width shape and position along the length of the wedge, it is possible to vary the heat flow into the sheet across the weld-track width to achieve virtually any temperature gradient across the weld-track. In this manner the amount of molten material across the weld-track width can be controlled, and the flow-rate and quantity of squeeze-out, as well as the reduction in thickness across the weld, can be optimized.